diff --git a/decode_reader.go b/decode_reader.go
index 19a3dbd..456120d 100644
--- a/decode_reader.go
+++ b/decode_reader.go
@@ -143,6 +143,9 @@ func (d *decodeReader) copyBytes(length, offset int) {
 		d.win[d.w] = d.win[i]
 		d.w++
 		i++
+		if i >= d.size {
+			i = 0
+		}
 		length--
 	}
 }
diff --git a/decode_reader_test.go b/decode_reader_test.go
new file mode 100644
index 0000000..cf293df
--- /dev/null
+++ b/decode_reader_test.go
@@ -0,0 +1,15 @@
+package rardecode
+
+import "testing"
+
+func TestCopyBytes_SourceIndexWraps(t *testing.T) {
+	d := &decodeReader{size: 8, win: make([]byte, 8)}
+	d.win[6] = 0xAA
+	d.win[7] = 0xBB
+	d.w = 4
+	// offset=6: source starts at (4-6)%8 = 6, wraps past 7 to 0
+	d.copyBytes(4, 6)
+	if d.win[4] != 0xAA || d.win[5] != 0xBB {
+		t.Errorf("first two bytes wrong: got %x %x, want AA BB", d.win[4], d.win[5])
+	}
+}